lodger leaving paid deposit how long notice of intention to move out

this is not for me but a potential new tenant. Currently she is lodging without a contract. Owner has a deposit and this person want out to move into my place. How long a notice should she give the owner? She is concerned that the owner will hold onto her deposit. I explained that deposit is for damages nothing else as not under a AST.

She really wants out to move into my place but my place may go if she cannot move in quick.

Advice gratefully taken.